This invention relates to the economical purification of water containing soluble and sparingly or partially soluble inorganic compounds using a two-stage membrane process with a unique recycle of "softened" membrane concentrate streams.
Water containing hardness compounds such as barium, calcium, magnesium, iron, silica, carbonate and bicarbonate, fluoride and sulfate is commonly found in surface water supplies such as lakes and rivers as well as underground water supplies such as water wells and aquifers and as aqueous industrial effluents and landfill leachates. This water is frequently purified by using water softeners in the form of "ion exchange resins", chemical softeners using the cold lime or hot lime softening process, reverse osmosis and nanofiltration membranes and/or distillation. Industry needs purified water containing low to very low concentrations of hardness compounds and of soluble inorganic compounds in order to supply their cooling towers, low-pressure and high pressure boilers, heat exchangers and various process uses. The pharmaceutical and electronics' industry as well as hospitals and laboratories require high purity waters which are almost completely free from inorganic compounds. The water purification processes listed above involve transferring the soluble water impurities to a resin bed which must be regenerated and/or disposed of at high cost, adding a large quantity of chemicals and generating a considerable volume of chemical waste in the case of lime softening, generating a substantial volume of reverse osmosis (RO) or nanofiltration (NF) membrane concentrates which must be treated further or disposed of at a large cost in the case of state-of-the-art RO and NF membrane processes and, in the case of distillation, incurring very high capital and/or operating costs.
Although membrane filtration processes such as reverse osmosis (RO) or nanofiltration (NF) have provided an effective and economically viable means for purifying water, these membrane processes in their current form are limited in the percentage of purified water produced, known as permeate or product recovery, since most of the soluble compounds are separated and concentrated into a smaller volume, typically 25-50% of the volume of the original water source. The membrane concentrate volume is too large and costly to dispose of, except in seawater desalination where the concentrate stream (also known as the reject stream) is returned to sea and in some other applications where there are no regulatory limits on the quantity of the reject stream discharged or the concentration of inorganic compounds contained therein. The main reason why further recovery of purified water from RO or NF membranes is not possible is the tendency of scale to form on the surface of the membranes as the concentration of scale-forming compounds such as calcium carbonate, calcium fluoride and silica is increased beyond their saturation values. This deposition of scale frequently results in a loss of purified water production (also known as loss of permeate flux through the membrane) and the eventual need for costly replacement of the membranes.
The use of chemical additives in the water supply such as acids to reduce the pH and inorganic or organic antiscalant compounds is practiced in the water treatment and membrane industry in order to provide some improvement in the water recovery and prevent scale formation. However, such improvement is only of limited extent since no anti-scalant is effective for all the contaminants and therefore they do not provide economically viable options for treatment of the entire water stream.
A survey of prior art shows the following patents:
U.S. Pat. No. 4,000,065 discloses the use of a combination of reverse osmosis (RO) and ultrafiltration (UF) to separate organic material from the aqueous stream. The contaminated aqueous stream is circulated from the high pressure compartment of an RO unit to the high pressure compartment of a UF unit, then to the low pressure compartment of the UF unit and then back to the high pressure compartment of the RO unit.
Japanese Patent 57-197085 discloses a filtration apparatus that comprises connecting UF apparatus and RO apparatus in series so as not to deposit scale on the RO membrane.
U.S. Pat. No. 3,799,806 discloses purification of sugar juices by repeated ultrafiltration and reverse osmosis purification steps.
U.S. Pat. No. 4,083,779 discloses a process for treatment of anthocyante extract by ultrafiltration and reverse osmosis treatments.
U.S. Pat. No. 4,775,477 discloses a process for extraction of cranberry presscake wherein the presscake is ground and subjected to microfiltration to remove colloidal high molecular weight compounds followed by reverse osmosis to recover a red-colored solution.
U.S. Pat. No. 5,182,023 discloses a process for removing arsenic from water wherein the water is first filtered to remove solids then passed through an ultrafilter, followed by a chemical treatment to adjust pH to a range from about 6 to 8. Thereafter, anti-scalants and anti-fouling materials are added before subjecting the water to reverse osmosis to provide a stream having less than about 50 ppb arsenic.
Japanese Patent 53025-280 discloses the separation of inorganic and organic compounds from a liquid by first using a reverse osmosis membrane and then using a second reverse osmosis membrane having a more permeable membrane such as a microporous or ultrafiltration membrane. Part of the contaminated liquid obtained from the first membrane is processed through the second membrane.
U.S. Pat. No. 5,501,798 discloses a high recovery water purification process involving the use of reverse osmosis followed by chemical precipitation of hardness compounds from the RO concentrate followed by microfiltration to separate precipitated solids and recycling of the "suspended solids' free concentrate" back to the RO.
All the above-referenced patents and available literature have been aimed at prevention of precipitation of inorganic scale and other foulants as the water is treated by reverse osmosis membranes since the purified water permeation rate deteriorates as scale and foulants build up on the surface of the membrane, with eventual irreversible loss of productivity and need for costly membrane replacement. Prior art teaches acidification (i.e. pH reduction) as means to reduce the potential of calcium carbonate scale formation, the addition of antiscalants such as polyacrylic acids and sequestering agents such as ethylene diamine tetracetic acid (EDTA) and sodium hexametaphosphate (SHMP) to reduce the scale formation potential due to barium sulfate, calcium fluoride, calcium and magnesium carbonate and sulfate and silica. However, these anti-scalant compounds are not sufficiently efficient to allow very high water recoveries and concentration factors to be achieved. Maximum recoveries in the presence of anti-scalants may be in the range 70%-80% based on the treatment of hard "well-water". The above-referenced patents also teach the separation of suspended solids existing originally in natural water sources and industrial effluents or the separation of chemically-precipitated compounds using ultrafiltration or microfiltration before reverse osmosis treatment. While removal of suspended solids by membrane filtration will prevent fouling of the RO membranes, it does not prevent concentration and eventual deposition of the initially soluble scale compounds, as the recovery of purified water is increased using RO. U.S. Pat. No. 5,501,798 teaches a high recovery process involving the use of a single stage reverse osmosis system, chemical precipitation and microfiltration (MF) and recycling of MF permeate to the RO membrane system to maximize the recovery of purified water. However, high water recovery from the process of U.S. Pat. No. 5,501,798 is not economically attractive since it requires the use of high pressure RO membranes for the entire purification process. The purified water recovery is also limited by the scale formation potential of the mixture formed by combining the "raw water" stream and the recycled, chemically precipitated and microfiltered RO concentrate. Furthermore, the recovery is limited by the maximum RO membrane system operating pressure of 1,000 psig. The process of U.S. Pat. No. 5,501,798 also suffers from the inherent disadvantage of high capital and operating cost associated with the chemical precipitation and suspended solids' separation step involving the use of microfiltration membranes or other separation means on a relatively large RO concentrate stream, which stream is necessarily recycled to minimize scale formation potential on the RO membrane.
Therefore, there is need for an improved, economical process for purification of water which provides high water recoveries, even in excess of 99%, while preventing formation of scale on the RO or nanofiltration (NF) membrane surfaces, and thus prolonging the useful life of such membranes.